Make appengine connection explicitly owned by impl/prod.

impl/prod/urlfetch.go

Index: impl/prod/urlfetch.go
diff --git a/impl/prod/urlfetch.go b/impl/prod/urlfetch.go
index 14c4bec14e931b0716f1596ef5e3027f4e9a55ac..e374df33c2cba9d520103589bc922a434b64182d 100644
--- a/impl/prod/urlfetch.go
+++ b/impl/prod/urlfetch.go
@@ -15,6 +15,6 @@ import (
 // useURLFetch adds a http.RoundTripper implementation to the context.
 func useURLFetch(c context.Context) context.Context {
 	return uf.SetFactory(c, func(ci context.Context) http.RoundTripper {
-		return &urlfetch.Transport{Context: ci}
+		return &urlfetch.Transport{Context: AEContext(ci)}
 	})
 }